The fate of dissolved carbon depends on the temperature and pressure under which it is held. Carbon dioxide is more soluble at low temperatures and high pressures. The fizz in a can of soda is produced by carbon dioxide that is held in solution under pressure and usually chilled. Opening the can releases the pressure, and the carbon dioxide escapes into the air as a mass of tiny bubbles. Warm the can before opening it and the escaping carbon dioxide will eject a froth of liquid.
When carbon dioxide dissolves in water it forms a solution containing free hydrogen ions carrying positive charge (H+), bicarbonate ions with negative charge (HCO3-), and carbonic acid (H2CO3). This makes the water acid. The reaction is reversible, so its products can change back into carbon dioxide and water.
Perturbing the natural cycle
Many marine organisms use bicarbonate to convert calcium (Ca) present in sea water into the calcium carbonate (CaCO3) of their shells.
The downward-point arrow indicates that calcium carbonate is insoluble and when the organisms die, their shells sink to the sea floor. The carbon dioxide can then react with water once more to form bicarbonate and carbonic acid. The water remains acid. If the shells sink below a certain depth, however, they enter a region where the low temperature and high pressure mean that the water contains more dissolved carbon dioxide in the form of carbonic acid. The calcium carbonate then dissolves. This produces bicarbonate, increasing the alkalinity of the water.
The depth at which calcium carbonate begins to dissolve is known as the carbonate lysocline and the depth at which the process is complete is the carbonate compensation depth (CCD). The carbonate lysocline is at an average depth of 11,484 feet (3,500 m) and the CCD is at 13,780-14,765 feet (4,200-4,500 m) in the Pacific Ocean and about 16,405 feet (5,000 m) in the Atlantic Ocean. This is the way carbonate sediments form, eventually to be changed into carbonate rocks, and it explains why this process does not occur in the deep ocean basins—the calcium carbonate dissolves before it reaches the ocean floor.
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